1. Field of the Present Invention
The present invention relates to an optical component, for example, a lens or a mirror, which is used in optical systems in a wavelength region of 400 nm or less (preferably 300 nm or less, such as an optical component for use in photolithography) and a method for manufacturing the same. More particularly, the present invention relates to a method of manufacturing optical components for use in UV photolithography that show a reduced surface loss in the UV region. The present invention improves the performance of the illumination lenses and projection lenses used in KrF and ArF excimer laser steppers using ultraviolet light sources with a wavelength of 300 nm or less.
2. Discussion of Related Art
An exposure apparatus known as a stepper has been used in photolithographic techniques where fine patterns of integrated circuits are exposed and transferred onto wafers made of semiconductors such as, for example, silicon. As Large Scale Integrated Circuits (LSI's) have become more highly integrated in recent years, the light sources used in such steppers have shifted to shorter wavelengths, i.e., from the g line (436 nm) to the i line (365 nm). More recently, the steppers have shifted to even shorter wavelengths, such as KrF (248 nm) and ArF (193 nm) excimer lasers.
Generally, the lens materials used in the illumination lenses or projection lenses of steppers consist mainly of optical glass with a high transmittance of the i line. On the other hand, in the case of KrF and ArF excimer lasers, synthetic fused silica and fluoride single crystals such as CaF.sub.2 (fluorite) are used instead of conventional optical glass.
Such optical components generally must have a transmittance of 99.5% or greater in the wavelength region used. Furthermore, a reduction of surface loss also is an especially important quality requirement in optical components.
It has been found that for optical components used in a short wavelength region of 300 nm or less, the surface loss cannot be reduced to 0.5% or less using conventional polishing methods or cleaning methods. As a result of long years of diligent research concerning the causes of this problem, the present inventors have ascertained the following facts.
(1) Surface loss includes loss other than scattering caused by the surface roughness.
(2) Some surface loss is caused by the absorption of metal residues, such as, for example, polishing agents.
Over a period of many years, the present inventors have conducted experiments in order to verify the above-mentioned facts.
A method using a low-pressure mercury lamp made of synthetic fused silica as a light source is generally known as an ultraviolet cleaning method. This light source emits ultraviolet light at 185 nm and 254 nm. Since the energy of this light source is greater than the bonding energy of most organic compounds, chemical bonds are broken when this energy is absorbed by organic substances, so that radicals and molecules in an excited state can be generated. Ultraviolet light at 185 nm is absorbed by oxygen molecules so that O.sub.3 is generated. O.sub.3 absorbs ultraviolet light at 254 nm and generates active oxygen. This active oxygen reacts with the radicals and excited molecules of organic substances, so that the organic substances are decomposed.
In order to obtain an optical material that has an internal transmittance of 99.5% or greater, it is necessary to manufacture a material that contains few impurities or structural defects that cause internal absorption in the optical material. Accordingly, synthesis by flame hydrolysis is used as a method for manufacturing synthetic fused silica with few impurities or structural defects. In this method, an Si compound gas (which serves as a raw-material gas), a carrier gas, which transports the Si compound gas, and gases which are used for combustion/heating (e.g., H.sub.2, O.sub.2 gas, etc.), are caused to jet from a burner, and fine particles of SiO.sub.2 that are produced in the flame are deposited on a target and simultaneously vitrified.
With respect to fact (1) discussed above, the relationship between surface roughness and transmittance was confirmed. FIGS. 1 and 2 show the respective relationships between surface roughness and the measured transmittance of experimentally manufactured optical components (.phi. 60.times.t10 mm parallel flat plates) at measurement wavelengths of 248 nm and 193 nm. Synthetic fused silica, which were all manufactured under identical conditions, were used as the measurement. Furthermore, the surface roughness was measured using an optical interference type surface roughness meter.
As shown in FIGS. 1 and 2, although the transmittance depends to a certain extent on the surface roughness, i.e., on the surface scattering loss, other factors also have an effect on the transmittance value.
This further shows that in addition to surface scattering, surface loss caused by absorption also largely influences the measurement of transmittance. It is thought that this absorption is caused by structural defects resulting from residual impurities and residual stress.
Even in cases where almost no metallic impurities such as CeO.sub.2 are detected and the surface roughness is less than 1 .ANG. RMS, the transmittance may still be lowered 0.5% or more compared to a theoretical transmittance. Thus, a problem remains.